1. Field of the Invention
This invention relates to selectively switching back and forth from an entry stand to an exit stand both acting as pivot stands at different time intervals in the mill during the several phases when rolling strip material.
2. Description of the Prior Art
A master speed regulation system for a multi-stand rolling mill is normally controlled by applying a speed reference to a pivot stand, whose speed remains constant at the previously scheduled rate, and cascading that speed reference, which is adjusted for automatic gauge control and tension control, to the stands upstream and downstream from the pivot stand depending on whether the mill is in a threading/tailing stage (low speed) or in a full run stage (high speed).
The pivot stand establishes coordination of the various stand speeds relative to each other. The choice of which stand to make the pivot stand effects the operation of the mill, that is, a pivot stand at the entry end of the mill improves the ability to thread the mill since the tension corrections are cascaded to the threading stand, and are not reflected to the upstream stand or stands through which the strip material has been threaded. Generally, the tension corrections cause the speeds of the downstream stands (having no strip) to be increased during the threading of the strip.
Once threading has been accomplished, however, and the mill is in its full run stage, a pivot stand at the exit end of the mill is more desirable. Under these conditions, the automatic gauge control system and the tension control system are prevented from changing the speed of the exit stand of the mill, resulting in a constant tension between the mill and the tension reel, and allowing the exit stand to operate at optimum velocity for production purposes.
It is undesirable to thread with the exit stand as the pivot stand. The natural tendency for each stand as the strip enters is a speed decrease due to load impact. This speed decrease is reflected through the tension regulators as a disturbance to all upstream stands. If the threading speed is sufficiently slow a stand may even reverse its direction and cause a strip break.
Historically, a compromise was developed especially in a five stand mill, where stand three was chosen as the pivot stand for the several stages of the mill. However, due to the differences in the mill characteristics between threading a mill and operating at the optimum running speed, a pivot stand at the entry end for threading and at the exit end for full run is still the most desirable arrangement since this setup results in optimum speed control for the several stages of the mill. In order to obtain this optimum speed control, it is desirable to be able to smoothly transfer back and forth between the entry stand acting as the pivot stand in the threading and/or tailing out of the strip and the exit stand acting as the pivot stand in the full run phase of the mill without disturbing the existing mill conditions.